But don't wait too long to start. The brain-boosting effects of exercise diminish rapidly after early middle age, say researchers working in the lab of Yu-Min Kuo, PhD, of Taiwan's National Cheng Kung University Medical College.

Kuo's team previously found that young brains create new brain cells and integrate them into existing brain networks. As animals get older, however, this process dramatically slows. And this slowdown in brain cell creation is linked to impaired memory and learning.

Can this age-related mental decline be reversed by exercise? To find out, Kuo and colleagues trained mice to run on exercise wheels, at 70% of their aerobic capacity, every day for five weeks. Control mice were not trained to exercise, although they did have exercise wheels.

Mice began exercising at age 8 months (early middle age for a mouse) or at age 12 months (midway to mouse old age, which is 24 months).

Sure enough, mice that worked out every day grew 2.5 times more new brain cells than couch potato mice. And in the exercising mice, far more of these new neurons survived, grew, and integrated into existing brain networks.

"Treadmill running not only increases the quantity but also enhances the quality of newborn neurons," the researchers report. "Chronic treadmill running alters the chemistries of middle-aged brains toward an environment resembling younger brains."

Mice that started exercise in early middle age did much better than mice that didn't start exercising until later middle age.

"Middle age represents an important turning point of life," the researchers say.

Interestingly, the brain changes seen in exercising mice weren't caused by a drop in stress hormones, as some studies predicted. Instead, the positive changes came from increased production of signaling molecules that promote brain cell growth and survival.

The findings appear in the November issue of the Journal of Applied Physiology.